The present invention relates to a process for the recovery of cobalt in the form of finely-divided and anhydrous salt from impure, aqueous, in particular nickel-bearing, solutions of cobalt, by exposing the aqueous solution to oxidation at an elevated temperature in the presence of ammonia and corresponding ammonium salt.
A process for producing a cobalt (II) salt by crystallizing it from a pure aqueous solution of the same is widely known. The aqueous solution from which the salt is crystallized is usually produced by dissolving a pure cobalt metal in an aqueous solution of the corresponding acid. A hydrous cobalt salt is obtained as a product of crystallization. It is not usually possible to obtain an anhydrous cobalt salt--e.g. cobalt sulfate--by crystallization performed at normal pressure and with solutions normally used.
It is highly advantageous to produce a pure and rapidly dissolving metallic cobalt powder suitable for the production of cobalt sulfate, as well as other cobalt chemicals and compounds, by the known pentammine process (W. Kunda, J. P. Warner, V. N. Mackiw, The Hydrometallurgical Production of Cobalt, Trans. Can. Inst. Mining Met., 65 (1972) 21-25); an impure, in particular nickel-bearing, sulfate solution or an ammoniacal sulfate solution is suitable as the raw material of this process. It can even be said that the dissolving of the cobalt powder produced by the pentammine process in a suitable acid and the crystallization or precipitation performed from the solution thus obtained represents the highest level of current technology for the production of pure cobalt chemicals and compounds from impure solutions of the type described above. It can further be noted that the most suitable and principal area of use for the cobalt powder produced by the pentammine process is obviously the production of cobalt chemicals and compounds. This is so because a finely-divided metal powder is usually not suitable for smelting operations and, furthermore, the cobalt powder produced by the pentammine process is usually too coarse or, owing to the production process, it has an over-high sulfur content for powder metallurgical applications.
An object of the invention is to provide a process for the production of cobalt sulfate from impure, in particular nickel-bearing, sulfate and ammoniacal sulfate solutions having the following advantages:
1. The cobalt sulfate is obtained directly without the necessity of producing metal as an intermediate product.
2. The cobalt sulfate is obtained directly in an anhydrous form without the necessity of resorting to special arrangements, such as calcination of a hydrous salt, autoclave crystallization, or the use of solutions with very high sulfuric acid concentrations.
3. The purity degree of the cobalt sulfate obtained as the product--as measured by the Co/Ni ratio--can easily be increased ten-fold.
4. The cobalt sulfate obtained as the product has a considerably smaller grain size.
5. The consumption of the reagent--especially the consumption of NH.sub.3 -- is considerably smaller.